Under conditions of increased hormone secretion (e.g., during reproductive functions), neurosecretory neurons in the hypothalamic paraventricular nucleus (PVN) develop characteristic patterns of electrical activity and neurohormone release. These patterns of activation are determined by the intrinsic electrical properties as well as by the synaptic organization of the hormone-secreting cells. Local synaptic circuits are crucial for the generation of patterned electrical activity, yet very little is known about the local synaptic regulation of neurosecretory neurons. The long-term objectives of this study are to characterize physiologically and anatomically the local synaptic organization of PVN magnocellular and parvocellular neurons, and to determine how local synaptic inputs regulate the electrical activity and hormonal/synaptic output of PVN neurons. The specific aims of this proposal are 1) to characterize pharmacologically and map topographically local synaptic inputs to PVN neurons from cells outside the PVN, 2) to determine whether PVN magnocellular and parvocellular neurons are synaptically coupled and the transmitters or hormones which mediate these interactions, and 3) to identify anatomically cell populations of the PVN which receive local synaptic inputs. During the course of these experiments, we will determine the effects of local synaptic inputs on the electrical activity of identified PVN cells. Experiments will be performed in rat hypothalamic slices using intracellular, whole-cell patch-clamp and extracellular recordings of PVN cells. Cells will be identified provisionally as magnocellular or parvocellular by their intracellular electrical characteristics. Glutamate microapplication will be used to stimulate selectively local neurons for topographic mapping of intra- and extra-PVN regions containing cells which provide inhibitory and/or excitatory synaptic inputs to recorded PVN neurons. Conclusive demonstration of synaptic coupling between PVN cells and local interneurons (PVN and non-PVN) will be accomplished with paired recordings and cross-correlation of action potentials and synaptic activity. Specific antagonists will be applied to determine the transmitters and transmitter receptors which mediate these local synaptic interactions. The effect of activation of local synaptic inputs on the patterned activity of PVN cells (e.g., phasic firing of vasopressin cells) will be analyzed. All intracellular/patch-clamp-recorded cells will be intracellularly labeled and immunohistochemically identified with antibodies directed against general neurophysin (to distinguish between magnocellular and parvocellular neurons), oxytocin, vasopressin or corticotropin-releasing hormone. These combined electrophysiological- anatomical studies will elucidate the local synaptic organization of the hypothalamic PVN, and will provide physiological data on the regulation of PVN cell output by local synaptic inputs.